Re: Fortran's Equivalence

[sampson@nosc.mil (Charles H. Sampson)]

In article <E8385t.9zE@thomsoft.com>, Keith Thompson <kst@sd.aonix.com> wrote:
> 
>2. If you're using overlays, watch out for default initializations.
>   For example, if a record type has default initial values for some
>   or all of its components, an address clause does not inhibit the
>   initialization unless the object is imported.  The same area of memory
>   could be initialized twice if you're not careful.
> 
     From something I've recently learned in another thread, I gather 
that such a program would be erroneous according to 13.3(13).  Appar-
ently it's o. k. to interpret the initializations as making the address 
invalid.

>3. If you're translating Fortran code to Ada (let's not get into
>   whether this is a good idea), and the Fortran uses EQUIVALENCE, you
>   should determine just how it's being used.  If the closest semantic
>   match to the particular usage of the EQUIVALENCE happens to be
>   Unchecked_Conversion, use Unchecked_Conversion.  This requires some
>   thought and analysis, which is why automated translation from one high
>   level language to another is impractical given the current state of
>   the art.
> 
>Just saying that a Fortran EQUIVALENCE should be translated into an Ada
>overlay misses some very important points.
> 
     Bless you, Keith.  This is very close to a point I've been trying 
to make in the CMS-2 world for years.  If you want to translate an over-
lay into Ada it's necessary to understand how the overlay is being used.  
The CMS-2 to Ada translator that I was responsible for was very conser-
vative; it refused to translate overlays at all, calling for user inter-
vention.  Unfortunately, users of CMS-2 translators want the translator 
to handle overlays.  They're all over the place and typically very dif-
ficult to figure out.  The result was that our translator would regu-
larly lose out to others that claimed to translate overlays by one or 
another simplistic approach.

     If development money had held out we intended to detect some com-
mon cases that were translatable.  And if cows had wings ...

				Charlie